ABSTRACT:

1. Cover crops can reduce selection pressure on current, effective herbicide options and they can also suppress weeds due to the biomass they produce.

2. Termination timing of the cover crop plays a role in and may influence the success of a cover crop program.

3. Cover crops and termination timing, in addition to adding the proper residual product to your termination herbicide, can help improve weed control.

THE IMPACT OF COVER CROPS ON WEED CONTROL

Cover crops not only play an important role in soil health, but also in weed control. Cover crops can reduce selection pressure on current, effective herbicide options, and they can also suppress weeds due to the large amounts of biomass they produce. Similar to canopy coverage, biomass will shade the soil and reduce weed emergence.

Beck’s Practical Farm Research (PFR)® team looked at various cover crop treatments across two termination timings: at planting and 21 days after planting (DAP). We saw greater visual control when we waited 21 DAP, but we also saw a reduction in stand. 

When terminated at planting, the soybean stands with no cover crop were 96K and the stands with a cover crop were 99K. However, when terminated 21 DAP, the stands with no cover crop were 107K and the stands with a cover crop were 77K. Though we saw a reduction in stand, soybeans can compensate for said reduction. 

WHICH COVER CROPS WORKED THE BEST?

The longer you wait to terminate the cover crop, the greater the increase in biomass and the harder it will be for the residual product to reach the soil surface. However, an increase in biomass can result in greater weed suppression. Cereal rye, at lower rates, produced less biomass than at higher rates, but still interfered with the herbicide application, making the worst of both worlds. Wheat resulted in the best weed control across the two application timings. Other cover crops, such as hairy vetch or rape seed, had poor growth in the fall due to dry weather. Therefore, the control from hairy vetch was most likely from the Group 15 herbicide and not the cover crop. Termination timing, plus the Group 15 herbicide you select, may influence the success of your cover crop program. 

SELECTING THE PROPER GROUP 15 HERBICIDE FOR COVER CROPS

Waterhemp residual control is very effective with Group 15 herbicides. However, like any residual herbicide, the product must reach the soil surface. Remember, the longer you wait to terminate the cover crop, the harder it is for the residual product to reach the soil surface because of the increased biomass. Products such as Warrant® contain an encapsulated acetochlor, making it very effective in high biomass scenarios. In the study below, the addition of Warrant to Roundup PowerMAX® provided the most constant level of control across the two application timings. Though terminating at 21 DAP made it more difficult for the herbicide to reach the soil surface, the increase in biomass still improved control. The key is that cover crops alone are not enough for season-long control, and adding the proper residual product to your termination herbicide can help improve control. 

CONCLUSION:

In addition to reducing selection pressure on current, effective herbicide options, cover crops can suppress weeds due to the large amounts of biomass they produce. Like canopy coverage, biomass will shade the soil and reduce weed emergence. Termination timing of the cover crop plays a role in, and may influence, the success of your cover crop program.
In addition, adding the proper residual product to your termination herbicide can help improve overall control.